The azimuthal torque around the slewing axis of a pod unit or azimuthal thruster has to be handled by an installed steering engine for all combinations of steering angles, propeller speeds and ship speeds.
The main causes of the azimuthal torque are:                The lateral force that is created due to an oblique flow toward the propeller together with the distance between the propeller and the vertical slewing axis. This distance forms one arm of a lever that is pivotal around the slewing axis.        At turning an oblique flow to the propeller blades will give a varying angle of attack over a complete turn at a given radius. This variation causes a torque that affects the total azimuthal torque.        The distance between the load center of the pod housing and the vertical slewing axis will together with the resulting lateral force give a torque that affects the total steering torque.        
A common way of reducing the azimuthal torque for pod units and azimuth thrusters is to place a fin with a wing profile abaft the slewing axis. The fin creates a lateral force due to the angle of attack that results, especially at turning of the pod unit. The lateral force gives rise to a torque that acts in a direction opposite to the sum of other torque contributions and therefore it reduces the maximum azimuthal torque.
At certain operating conditions, a fin with a wing profile placed in the slip stream of a propeller may generate a forward directed force, which is greater than the total drag on the fin that acts in the opposite direction. Thereby, this regain of the rotational energy in the slip stream will give a positive thrust contribution that increases the efficiency of the pod unit. The distance between the slewing axis and a center of the lateral forces acting on the fin forms a second arm of the lever.
The use of such a fin is disclosed in WO 2005/012075 A1 (Rolls-Royce Marine AS) and JP 2004090841 (A) (Kawasaki Heavy Ind. Ltd.), for example. However, a fin will project a comparatively large distance from the pod body, which causes an increased risk of grounding. A conventional fin will also give disadvantages in the form of more complicated handling and transportation of the pod unit on docking the ship, for example, and increase the dimensioning loads for mainly pod housings and slewing bearings. In addition, the complex shape (the wing profile) may cause the manufacturing costs to be relatively high.
JP 2009214650 (A) (Universal Shipbuilding Corp.) discloses an invention, the object of which is to provide a pod type propulsion unit capable of reducing propulsion resistance without developing a separation phenomenon in a liquid flow at manufacturing cost suppressed to a low level by a simple configuration. This object is stated to be achieved in that the pod type propulsion unit comprises a propeller, a pod body, and a strut, wherein rectangular-plate vanes (current plates) are fixed to the side surface of the pod body so as to be disposed parallel to the axial direction of the pod body and in the direction normal to (the same as the radial direction of) the side surface of the pod body. The amount of projection of the vane is 40% or smaller of the radius of the propeller, so that the projection is extremely small compared to conventional known fins. Further, from WO 01/54973 there is known a POD arrangement having fins, but not for the purpose of reducing torque or reducing resistance, but to improve cooling.